A camera may comprise photosensor, such as a CCD (charge coupled device) or CMOS (complementary metal Oxide on Silicon) photosensor and optics that collects and focuses light from a scene that the camera images onto the photosensor during an exposure period of the camera to acquire an image of the scene. The photosensor generally comprises an array of “horizontal” rows and “vertical” columns of light sensitive pixels that register the light focused by the camera optics onto the photosensor. Amounts of light registered by the pixels are determined to provide the image of the scene.
A pixel in the photosensor registers incident light from a feature in the scene that the optics focuses on the pixel by accumulating negative or positive electric charge provided by electrons or holes respectively from electron-hole pairs that the incident light generates in the pixel. The charge provided by electrons or holes from electron-hole pairs may be referred to generically as “photocharge”. Camera photosensors are typically configured so that their pixels accumulate electrons, conventionally also referred to as “photoelectrons”, and thereby negative photocharge originating from electron-hole pairs, rather than holes, and thereby positive photocharge to register incident light. Converting an amount of photocharge that a pixel in the photosensor accumulates responsive to incident light to a voltage, hereinafter also referred to as a readout voltage, provides a measurement of an amount of the incident light that the pixel registers. A collection of the readout voltages referred to as a frame of the photosensor may be used to provide and image of the scene. A CMOS photosensor may include circuitry in each pixel of the photosensor to convert photocharge to readout voltage. An interline CCD photosensor transfers photocharge accumulated by each of the pixels along an array of photocharge storage capacitors to readout circuitry that converts the photocharge to a readout voltage. Converting photocharge accumulated by a pixel to a readout voltage may be referred to as “reading” the pixel, or “reading” the photocharge.